He went on: “It’s healthy for people to hold alternative views on the future direction of the party.

“What’s toxic though is for abuse, threats and aggressive language to be considered acceptable – or the norm.

“Labour is in danger of becoming the new ‘nasty party’ if this behaviour continues unchecked.”

Writing in the Daily Mirror, Prentis also warned against “chasing ideological purity” before saying: “There can be no place for witch hunts against Labour MPs, councillors and party staff by the left or right of the party.

“But neither can we accept the grotesque farce of Labour MPs heckling their own democratically elected leader in the House of Commons.”

In a letter obtained by HuffPostUK, Lillis wrote: “Over the past few weeks I have become increasingly alarmed by both the public and private concerns being raised by your team regarding the conduct and integrity of the General Secretary and the Labour Party staff during this leadership contest.”

The Labour leader’s team have alleged that McNicol had given Smith – but not Corbyn - advance sight of the questions at the first party hustings in Cardiff.

Corbyn supporters have also complained that the Smith campaign had unfair earlier access to members’ emails and other data.

Unison’s backing of Corbyn comes days after Unite, the UK’s biggest union, also announced its support for the current Labour leader.

Corbyn said: “I am proud to have the support of Unison members. Their incredible work, against the backdrop of cuts, privatisation and outsourcing of public services, keeps the services we all rely on running - from the NHS to local government.”

Former Shadow Work and Pensions Secretary Owen Smith has secured the nomination of GMB, which carried out a full ballot of its members.

A spokesperson for Smith’s campaign said: “Owen agrees with Dave Prentis that Labour needs to be a united opposition.

“That is why, over the course of this campaign, he has been setting out the ideas and vision to bring our party back together and provide a radical, credible alternative to this right-wing Tory government.”

Members of affiliated unions are one of three sections of the Labour leadership electorate, alongside registered supporters and party members.